1 Field of the Invention
The present invention relates generally to a process for developing a positive electron-beam sensitive resist film. More particularly, it relates to a process for developing a resist of a copolymer of methacrylonitrile and methyl alpha-chloroacrylate. The resist after being exposed in an electron beam and developed by the process disclosed, produces a vertical resist profile suitable for use in high resolution submicron dimension electron beam lithography.
2. Description of the Prior Art
The application of electron beam techniques to the art of semiconductor fabrication has enabled great strides to be made in reducing the minimum line width and thus the size of a pattern which can successfully be manufactured. This has been accomplished both through progress in the technology involved with the perfection in the precision of the electron beam system itself and in the progress which has been made in the technology concerned with the pattern defining medium or resist material.
In electron beam microfabrication, a substrate, which may be, for example, silicon dioxide, silicon, glass or chromium plated glass, is further coated with a layer of polymer resist material and the resist is patterned by changing the solubility of the polymer with an electron beam. Subsequently, the mask is "developed" by dissolving away the unwanted area of polymer utilizing a suitable solvent material and the resultant pattern is used as a mask either for plating, chemical etching, ion etching or ion implantation. This electron-beam lithography (EBL) is an integrated circuit production technique which utilizes a polymer resist to delineate circuit patterns for monolithic circuits.
When polymers of the required type are irradiated with an electron beam, the molecular structure is affected such that some of the polymer molecules are excited or ionized by the beam. This excitation causes some of the resist molecules to cross link with other molecules in the polymer structure and others to degrade or undergo scission. The predominant manner in which such a polymeric material reacts to exposure to an electron beam has led electron resists to be classified into two main categories. Thus, a polymer which becomes predominantly gelled or crosslinked, and thereby decreases its solubility after irradiation, is termed a negative resist. Conversely, if the scission process predominates and the solubility of the polymer increases after irradiation, it is called a positive resist. The resist to be developed by the process of the present invention is a positive resist.
A suitable electron resist must have various physical and chemical properties which allow it to fulfill the requirements for electron beam fabrication. The polymer material involved must be sensitive to an electron current of a fairly low value or the resist sensitivity itself will be the limiting factor on the writing speed and line width which can be achieved. The resist medium must be capable of a high resolution or resist contrast compatible with that achieved in writing and etching techniques utilizing the high resolution capability available with electron beam technology. The resist must also have the ability to adhere satisfactory to a variety of substrates used in different microfabricated applications. The medium also must be able to withstand normally acid, base, and ion etching processes and should not be sensitive to small daily process variabilities.
Poly (methyl methacrylate) (PMMA) has been the standard positive e-beam resist material. It is capable of high resolution but is not sensitive enough for commercial device production using direct-write e-beam lithography. The e-beam sensitivity of PMMA is 1.1.times.10.sup.-4 Coulombs per cm.sup.2, while sensitivities in the 10.sup.-5 to 10.sup.-6 range are desirable for EBL production; resolution, however, cannot be sacrificed for sensitivity. Resolution is stipulated by the minimum obtainable line width and line space for the resist. In this connection, PMMA is capable of resolving 1 micron lines and spaces.